In ultrasound imaging, which measurement is used to determine the pixel location in the image?

• A. How long it took the echo to be received
• B. How strong the echo was
• C. The transmitter frequency
• D. The speed of sound in tissue

Answer: (A)

In ultrasound imaging, where a pixel sits along the depth axis is determined by how long it takes for the emitted sound pulse to return as an echo. This time-of-flight is converted into depth using the known speed of sound in tissue (depth ≈ c × t / 2, since the sound travels to the reflector and back). So the measurement used to determine pixel location is the round-trip time of the echo. The echo’s strength affects brightness, the transmitter frequency affects resolution and penetration, and while the speed of sound in tissue is needed to convert time to depth, it’s the time measurement that sets the position.